Reference is made to U.S. Pat. Nos. 3,774,186 and 3,863,076.
Typical particle detectors include an exciter lamp and a photocell normally shielded from direct lamp light and responsive to light scattered from smoke or other fluid borne particles in the light beam to trigger a threshold circuit which then produces an alarm signal actuating a horn or other local or remote alarm indicator.
When incandescent lamps are used as light sources they are shortlived and subject to unpredictably early failure rendering the detector inoperative, often unnoticed. It is therefore highly desirable to include in the detector a circuit for sensing lamp failure and giving an audible or visible alarm of the trouble condition. Incandescent lamps are more reliable and long lived if operated below their rated voltage, and light emitting diodes are substantially more reliable and are considered to have infinite life for practical purposes. But while the need for sensing and indicating lamp failure is greatly reduced it is still desirable to provide a trouble alarm as well as the primary alarm of smoke, for example.
Accordingly it is the object of the present invention to take advantage of the increased reliability and life of lamps, particularly LEDs, by providing a trouble or lamp failure alarm circuit which is greatly simplified and reduced to a low cost so that it can be more easily afforded.